In nuclear power plants ionic impurities are normally removed from the water in the reactor primary system with the aid of ion-exchange filters containing ion-exchange resin. The impurities consist mainly of corrosion products from the construction material in the primary system, such as ions of or containing iron, manganese and chromium, which have become radioactive when the water containing these corrosions products passes the reactor core. The water in the primary system may also contain impurities in the form of fission products of reactor fuel, such as ions of caesium and strontium which have leaked out into the water. Even water outside the primary system, such as the water from floor drains and waste-water pipes, is normally subjected to purification in filters containing ion-exchange resin to remove radioactive impurities. After some time in use, the ion-exchange resin is spent and must be removed, solidified and safely stored.
Various methods are known for solidifying ion-exchange resin. One method is to mix the ion-exchange resin, considerably diluted with water, with cement in storage containers, usually concrete chills which are a cubic meter in size, and to allow the cement to solidify. Another method is to dry the ion-exchange resin, mix it with bitumen, and store it in sheet-metal barrels. Yet another method is, after drying the ion-exchange resin, to cast it in a plastic such as epoxy plastic or polyurethane plastic.
European patent application No. 0 096 342 describes a method of decomposing spent ion-exchange resin and solidifying the decomposition product with an inorganic binder. The object in this case is to effect a solidified product, entirely of inorganic material, having as little volume as possible. The ion-exchange resin is subjected to decomposition in an aqueous liquid by means of oxidation, "wet combustion", in the presence of an acid, such as sulphuric acid or nitric acid, and iron (III) ions. The oxidation agent may be hydrogen peroxide. The iron (III) ions functions as catalysts for the decomposition process. The temperature may be maintained at 80.degree.-100.degree. C. during decomposition and the pressure at atmospheric pressure. Decomposition of the ion-exchange resin releases volatile substances, primarily carbon dioxide, which is removed from the reaction vessel, while the aqueous phase will contain sulphate ions or other negative ions from the acid, from the catalyst and from groups included in the ion-exchange resin active at the ion-exchange reactions. In order to prevent the acid aqueous phase from corroding the equipment used during continued processing of the water phase, this phase is neutralized with sodium hydroxide so that the liquid will contain sodium salts, generally in the form of sulphate, as by far the dominant component weight-wise. The liquid is then subjected to concentration and evaporation to leave an inorganic material containing vast quantities of non-radioactive sodium salts. In the known case, this remainder is solidified in silicate, such as sodium silicate, or in cement. It is also known to effect decomposition using oxygen as the oxidation agent under high pressure, or with an oxidizing acid such as a mixture of sulphuric acid and nitric acid.